The present invention relates to a clock control circuit that performs spread spectrum.
It is known that electromagnetic noise radiated from a semiconductor device affects other devices. A spread spectrum clock is generally used to satisfy an EMI regulation value that defines a regulation value related to the noise. The spread spectrum clock is a clock to which a frequency swing is added. As shown in FIG. 21, it is possible to perform control so as to lower a peak value of a spectrum of a clock by using the spread spectrum clock. The spectrum of the clock is determined by a modulation waveform for adding a frequency swing.
Japanese Patent Laid-Open No. 2004-207846 (Patent Document 1) discloses a control method for temporally changing a period of frequency variation of the spread spectrum clock by temporally changing a period of amplitude of a voltage waveform input into a VCO. Peaks of the clock are flattened by controlling as described above.
Japanese Patent Laid-Open No. 2004-208037 (Patent Document 2) discloses a control method for temporally changing a frequency variation width of the spread spectrum clock by temporally changing an output amplitude of a voltage waveform input into a VCO. Peaks of the clock are flattened by controlling as described above.